Mass spectrometry is an analysis in which a sample is ionized and a ratio of a mass of a sample or sample fragment ions to electric charge (hereinafter, the ratio is referred to as “m/z value”) is measured so as to analyze a molecular weight of the sample. As the mass spectrometry, there is known matrix-assisted laser desorption/ionization mass spectrometry (MALDI) in which a low molecular weight organic compound referred to as a matrix is mixed with a sample and laser is irradiated to the mixture so as to ionize the sample. According to the method, laser energy absorbed in the matrix is transmitted to the sample, so that it is possible to ionize the sample satisfactorily.
MALDI allows ionization of a thermally instable substance and a polymer substance, so that this method allows “softer” ionization of the sample than other ionization techniques. Therefore, this method is widely adopted for mass spectrometry of various substances such as biopolymer, endocrine disrupter, synthetic polymer, metallic complex, and the like.
However, MALDI uses an organic compound matrix, and as a result it may be difficult to analyze sample ions due to related ions derived from the matrix. Specifically, in case where the organic compound matrix is used, there are observed matrix-related ions such as (i) matrix molecule ions, (ii) cluster ions caused by hydrogen bond of matrix molecules, (iii) fragment ions generated by decomposition of matrix molecules, and as a result it is often difficult to analyze the sample ions.
As such, various techniques for avoiding disturbance of the matrix-related ions have been conventionally proposed. Specifically, there is known a technique in which the matrix molecules are fixed so as not to generate the matrix-related ions. For specific example, there is disclosed a technique in which a matrix such as a-cyano-4-hydroxy cinnamic acid and cinnamamide is fixed to sepharose beads. Further, there is disclosed a technique in which a self-assembled monomolecular membrane of methyl-N-(4-mercaptophenyl-carbamate) serving as a matrix is formed on a surface of gold that is a target. Furthermore, there is disclosed a technique in which 2,5-dihydroxy benzoic acid (DHB) serving as a matrix is fixed in a silicon polymer sheet through sol-gel processing. However, the method in which the matrix molecules are fixed in the foregoing manner raises such a problem that detection sensitivity and durability are insufficient in practical use. Further, the method also raises such a problem that it is impossible to avoid a noise caused by the fragment ions.
For that reason, a technique using no matrix has been proposed recently. Specifically, there is disclosed a technique in which a semiconductor substrate having porous surfaces (in the document, referred to as “porous light-absorbing semiconductor substrate) is used as a sample target (see Patent Document 1 for example). The sample target is obtained by treating a sample support surface of the semiconductor substrate so that the sample support surface has a porous structure, i.e., a finely bumpy structure. This Document reports that: in case where laser light is irradiated to a sample applied to the sample support surface, a high molecular weight substance is ionized even when there is no matrix. This method is referred to as “DIOS” (Desorption/Ionization on Porous Silicon).
Further, the inventors of the present invention found a technique that allows ionization without using any matrix. According to the technique, by adopting (i) a sample target having as a sample support surface a surface having a finely and regularly bumpy structure of an order ranging from nanometer to several dozen micrometer which is produced by lithography or (ii) a sample target having a sample support surface whose face with a finely bumpy structure of an order ranging from nanometer to several dozen micrometer is coated with metal, it is possible to improve ionization efficiency and stabilize ionization compared with the conventional technique using no matrix (see Patent Document 2 for example).
Further, the inventors of the present invention studied a sample target having, as a sample support surface, a surface made of various materials each having a bumpy structure of a submicrometer order, and they found it possible to carry out ionization without any matrix also by using, as one of the aforementioned materials, a sample support surface that is obtained by coating porous alumina with gold or platinum (see Non Patent Document 1 for example).